A large number of methods are known nowadays for data transmission in digital radio communication systems, RFID systems being one example. In accordance with the particular system, the data transfer takes place between a transponder, implemented, for example as a radio tag, and a reading device. At low communication frequencies, inductive fields provide the transmission medium, while electromagnetic fields are used at higher frequencies. In systems of this type the inductive or electromagnetic field is modulated by information. Modulation consists in a modification of the signal parameters of a carrier frequency, that are the amplitude, the frequency or the phase, in accordance with a modulating signal, known as the baseband signal.
The field has to be demodulated in order to regain the baseband signal, i.e. the real information. Since both the transponder and the reading device contain a source of information, and therefore an exchange of information in both directions takes place, both pieces of equipment often contain both a modulator and a demodulator.
To regain the information, an antenna is now used to extract electrical power from the field. An inductive current arises in the antenna coil as soon as it comes into the region of an electromagnetic field. The modulation of the field is thus converted into the modulation of a proportional electrical current. The information can now be detected in the changes of this current over time. Parallel circuits, also known as shunt circuits, are used to detect the current.
A digital radio communication system involves, amongst other things, a highly dynamic electromagnetic field. In other words, depending on the distance between the transmitting and receiving equipments, the information is transmitted in a form of a highly variable field. The modulated field therefore has a rapidly changing modulation component, representing the information, and a slower component resulting from changes in the distance between the transmitter and receiver, or from changes in the transmission medium.
After converting the field into a proportional current, it is therefore necessary to normalize the current before being able to process the information contained in the current any further, so that subsequent circuit elements are not loaded beyond appropriate limits.
Normalization of this type is referred to as dynamic compression. In the past, one way of achieving dynamic compression has been to use a number of diodes in series. Another way of achieving compression has been to use a specialized analogue/digital converter, in which two demodulation units are provided with variable sensitivity modules, thereby switching between several outputs.